Worshipper – “Ship” [Campaign 2, Mission 4]

If the Elves only have a couple of trees left that are about to finish you off, you can build a huge army with three Elven castles, led by an Elven hero, and throw it at the enemy first (that's what I did in the third mission). I started building these castles from the very beginning. I ended up with a large army, but I didn't need it. Agrail leveled up on neutral creatures to level 30 and got the Call of Urgash at level 28. He also had a large army.